The present invention relates generally to information retrieval from electronic storage devices, and more particularly, to a method and system for private information retrieval from an electronic storage device using one or more servers.
With the advancement of Internet communications, the need for private information retrieval is increasing. For example, if a company maintains an Internet website with a complete listing of patents, competitors may be interested in searching those patents when introducing competitive products or services. Without privacy from the inquirer, the company can monitor database inquiries into its patent portfolio and gain knowledge as to the identity of the patents in which the competitors are interested. Thus, there is a need for privately retrieving selected information from an electronic storage device such that the information is not revealed to the storage device.
Private information retrieval (xe2x80x9cPIRxe2x80x9d) schemes allow a user to retrieve information from an electronic storage device while maintaining the privacy of the queries from the electronic storage device. As used herein, xe2x80x9celectronic storage devicexe2x80x9d refers to any processing system that stores information that a user at an inquiring processor may wish to retrieve. Moreover, the terms xe2x80x9celectronic storage devicexe2x80x9d and xe2x80x9cdatabasexe2x80x9d will be used interchangeably and should be understood in their broadest sense.
The least sophisticated method for performing a private information retrieval is to request a copy of the entire database. This, of course, hides the particular information sought by the inquiring processor, but requires an impractical exchange of information between the database and the inquiring processor. Moreover, when a database is large and its contents frequently change, the inquiring processor must receive frequent and possibly costly updates whenever the database changes.
Another PIR method requires replication of the database at alterative sites that do not communicate with one another. This replication technique is disclosed in B. Chor and O. Goldreich et al., xe2x80x9cPrivate Information Retrieval,xe2x80x9d 36th Annual IEEE Symposium On Foundations Of Computer Science (1995), A. Ambainis, xe2x80x9cUpper Bound On The Communication Complexity Of Private Information Retrieval,xe2x80x9d in the proceedings of 24th International Colloquium on Automata Languages and Programming, Volume 1256 (1997), B. Chor and N. Gilboa, xe2x80x9cComputationally Private Information Retrieval,xe2x80x9d 29th Symposium On The Theory Of Computing, and R. Ostrovsky and V. Shoup, xe2x80x9cPrivate Information Storage,xe2x80x9d 29th Symposium On The Theory Of Computing. To assure privacy of the requested information, this method requires replication of the database at alternative sites that do not communicate with one another.
An improved PIR method that does not require replication of a database at alternative sites is disclosed in pending U.S. patent application Ser. No. 09/087,893, entitled xe2x80x9cMethod And Apparatus For Private Information Retrieval From A Single Electronic Storage Device,xe2x80x9d now U.S. Pat. No. 6,167,392 and E. Kushilevitz and R. Ostrovsky, xe2x80x9cReplication Is Not Needed: Single Database, Computationally Private Information Retrieval,xe2x80x9d in the proceedings of 38th Annual IEEE Symposium on the Foundations of Computer Science (1997). This improved PIR method privately retrieves selected information from a portion of or an entire database without revealing to the database the address of the selected information. The method conceals the address of selected information in an encoding function that hides or conceals the address from the database. The database applies the encoding function to the relevant portion of database, and returns the result back to an inquiring processor. A disadvantage of the method is the attendant complexities associated with determining, at the inquiring processor, an encoding function that can conceal the address of the selected information from the database.
Another method, which uses a plurality of servers, is disclosed in D. Beaver, xe2x80x9cCommodity-Based Cryptography,xe2x80x9d in the proceedings of 29th Symposium Of Theory Of Computing (1997), pages 446-455. The Beaver method describes an oblivious transfer protocol for private information exchange between an inquiring processor and a database. The inquiring processor and the database receive from a plurality of servers commodities, which the inquiring processor and the database use to privately exchange information with each other. The Beaver method, however, has two disadvantages: First, it does not reduce the attendant complexities associated with privately retrieving selected information from a database. Second, to assure private information retrieval, the Beaver method requires that at least half of the server do not communicate with one another, a requirement which may not be practically enforced against servers.
Therefore, it is desirable to have a method and system for private information retrieval which overcome the disadvantages of the prior art.
Methods and systems consistent with the present invention privately retrieve selected information from a database, which includes, for example, N bits of information, by determining, at a server, a first commodity and a second commodity, communicating the first commodity to an inquiring processor and the second commodity to the database, and retrieving the selected information from the database based on the first commodity and the second commodity such that the selected information is not revealed to the database. The first commodity may include, for example, a random address in the database and trapdoor information for decoding query results received from the database by the inquiring processor. The second commodity may include, for example, a query for encoding the random address. The query may be determined using any conventional private information retrieval method or system.
The inquiring processor determines an address offset based on the random address and the address of selected information in the database, and sends the address offset to the database. The database cyclically shift its contents according the address offset, and executes the query on the cyclically shifted contents. The database then sends the result of the query to the inquiring processor, which extracts from the result the selected information in the database.
Methods and systems consistent with the present invention privately retrieve selected information from a database by combining commodities from a plurality of servers such that the selected information is not revealed to the database. A first server determines a first commodity and a second commodity, and a second server determines a third commodity and a fourth commodity. The first server communicates the first and the second commodities to an inquiring processor and the database, respectively. The first commodity may include, for example, a first random address in the database and trapdoor information for decoding query results received from the database by the inquiring processor. The second commodity may include, for example, a first query for encoding the first random address.
Likewise, the second server communicates the third and the fourth commodities to the inquiring processor and the database, respectively. The third commodity may include, for example, a second random address in the database and trapdoor information for decoding query results received from the database by the inquiring processor. The fourth commodity may include, for example, a second query for encoding the second random address, respectively. The second query may also be determined using any conventional private information retrieval method or system.
The database then combines the first and the second queries as follows: The inquiring processor determines a first address offset based on the first random address and the address of selected information in the database. The inquiring processor determines a second address offset based on the second random address and the first address offset, and sends the second address offset to the database. The database executes the first query, and stores the result of the first query. The database then cyclically shifts its contents in, for example, 1 bit increments N times, reexecutes the first query on each cyclically and incrementally shifted content, and stores the results of the N executions in the order of execution.
The database then cyclically shifts the stored results of the executions according to the second address offset, executes the second query on the cyclically shifted stored results, and sends the result to the inquiring processor. The inquiring processor then extracts the selected information in the database from the result received from the database.
In addition, methods and systems consistent with the present invention may verify the validity of a commodity determined by a server. The server identifies an address in the first database, and determines a private information retrieval query for encoding the address. It sends the address to an inquiring processor and the query to the first database. The first database executes the query on a second database, whose contents are known to the inquiring processor, and sends the result of the query to the inquiring processor. The inquiring processor decodes the result of the query to verify the validity of the query based on the identified address.
Alternatively, methods and systems consistent with the present invention may verify the validity of a commodity when privately retrieving selected information from a first database based on that commodity. A server identifies a first address in the first database, and determines a private information retrieval query for encoding the first address. It sends the first address to an inquiring processor and the query to the first database. The inquiring processor identifies a second address in the first database that includes selected information, which the inquiring processor desires to retrieve. The first database executes the query on a second database whose contents are known to the inquiring processor, and sends a first result of the query to the inquiring processor.
The first database then combines the first and second databases, executes the query based on the combined first and second databases, and sends a second result of the query to the inquiring processor. The inquiring processor decodes the first and the second results received from the first database, and extracts the selected information from the first and the second decoded results.
Methods and systems consistent with the present invention have several advantages over the prior art. First, the present invention reduces the complexity and the extent of communication between an inquiring processor and a database. The complex task of determining private information retrieval queries for encoding database addresses is delegated to one or more servers, and can be performed prior to, and independently of, an inquiry by the inquiring processor. Second, methods and systems consistent with the present invention can assure private information retrieval even though all but one of a plurality of servers communicate or collude with each other.
The description of the invention and the following description for carrying out the best mode of the invention should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the description for carrying out the best mode of the invention, show several embodiments of the invention, and together with the description, explain the principles of the invention.